摘要:

PbCrO3has been synthesized in the high‐pressure ``belt'' apparatus from PbO and CrO2at 1150°C and a pressure in excess of 50 kbar. Single‐crystal and powder x‐ray diffraction at room temperature show the compound has the cubic perovskite structure witha0=4.00 Å. No deformation of the cubic structure was discerned in neutron diffraction patterns at 77° and 4.2°K. At low temperature the magnetic moments of the chromium atoms are ordered in the antiferromagneticG‐type structure in which the spins on each Cr are antiparallel to those of the six nearest neighbors. Assuming the magnetic form factor of chromium can be approximated by that of Cr3+, the magnetic intensities give a moment of 1.9 &mgr;Bper chromium atom, approximately the spin value for Cr4+(3d2). The temperature dependence of the (111) magnetic peak was measured and the results fitted to a Brillouin function withTN≈240°K. The magnetic susceptibility does not show a maximum at the Ne´el point but does obey a Curie‐Weiss law aboveTN, consistent with the neutron‐diffraction results.

ISSN:0021-8979    

DOI:10.1063/1.1709698

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The distribution of magnetic ions and the crystallographic parameters remain essentially constant throughout the system Hg1−xCdxCr2S4. A study of the magnetic properties of this system was therefore undertaken in order to further elucidate the origin of the large difference in the magnetic properties already reported on the endpoints. The Curie temperature was found to increase nonlinearly from 36.0° to 84.5°K asxvaries from 0 to 1. The low‐temperature F‐AF (ferromagnetic‐antiferromagnetic) transition and metamagnetism of HgCr2S4penetrate well into the system, up tox≃0.37 at which the F‐AF transition temperature approaches 0°K.The distribution of magnetic ions remains constant, but the crystallographic parameters vary in the system Zn1−xCdxCr2Se4. Nevertheless, since ZnCr2Se4is an antiferromagnetic with large positive &thgr;, while CdCr2Se4is a ferromagnet, a magnetic study of this system provides further evaluation of the theory of the ferromagnetic spinels. In addition, the evaluation of the nonlinear variation of ferromagnetic Curie temperature from 22° to 129.5°K, observed forx=0.35 to 1, permits qualitative evaluation of the exchange parameters of ZnCr2Se4. The Ne´el temperatures were found to be ≃20°K for the whole antiferromagnetic region of the systemx=0 to 0.35. F‐AF transitions in zero field and/or metamagnetism are also observed over a large compositional region.

ISSN:0021-8979    

DOI:10.1063/1.1709699

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Single crystals of CdCr2Se4were grown for the first time. Individual pills of CdSe and CrCl3, in close contact, were heated around 700°C for three days. Perfect octahedra up to 2 mm in size grew in the CdSe pill by liquid transport. Formation of CdCr2Se4occurred only in the presence of platinum. The technique and its reactions are described in detail.

ISSN:0021-8979    

DOI:10.1063/1.1709700

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Previous EPR measurements on Gd3+pairs in LaCl3have shown that the interactions between nearest and next‐nearest neighbors are accurately described by isotropic bilinear exchange and magnetic dipole coupling, and they indicated that the ferromagnetic ordering of GdCl3at 2.2°K is due to dominant next‐nearest neighbor exchange. Similar measurements have now been made using EuCl3as the host lattice. The results are similar to those in LaCl3, and since the lattice spacing in EuCl3is very close to that of GdCl3, it is now possible to use the interaction constants measured for the Gd3+pairs to make quantitative predictions of magnetic bulk properties of GdCl3. Excellent agreement is found for the high‐temperature susceptibility; for the specific heat, there is a small discrepancy. The measured interaction constants are close to one of a set of values recently used by Marquard to predict some highly anomalous spin‐wave spectra for GdCl3.

ISSN:0021-8979    

DOI:10.1063/1.1709701

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The NaCl‐type compounds Eu1−xGdxSe form a continuous series of solid solutions between the ferromagnetic insulator EuSe and the antiferromagnetic metal GdSe. We have studied the temperature dependence of electrical resistivity &rgr;, in Eu1−xGdxSe single crystals and polycrystals between 2.5° and 300°K; we find, in qualitative agreement with earlier observations, for smallx, an exponential increase in &rgr; similar to normal semiconductors but a very sharp drop in &rgr; below the magnetic transition temperature. The resulting peak in &rgr; decreases sharply with applied magnetic field strength and increasing Gd concentration. The temperature dependence of the Hall effect in single‐crystal samples of Eu0.95Gd0.05Se indicates as origin of the resistivity anomaly an unusually large effect of the magnetic order on the carrier mobility. Preliminary thermoelectric power measurements support this conclusion. The possibilities of describing strong interactions between conduction electrons and localized spins by scattering theory and by magnetic self‐trapping of electrons in the spin lattice are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1709702

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Magnetic and electrical measurements are reported on single crystals of CdCr2Se4. The measurements include ferromagnetic resonance linewidth and magnetic moment vs temperature, magnetic anisotropy constantK1andg‐factor at 4.2°K, and the sign of the Seebeck coefficient at 300°K. A theoretical calculation ofK1at 4.2°K is also presented.

ISSN:0021-8979    

DOI:10.1063/1.1709703

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Measurements of the specific heat have been made on three samples of dysprosium aluminum garnet in the critical point region of 2.5°K, using large optical‐quality single crystals and temperature steps down to 50 &mgr;deg. The results for all three samples were very similar but they could not be fitted unambiguously to any of the presently proposed singular functions. In all cases, rounded maxima 2 to 3 mdeg wide were found, with peak values between 4.2 and 5.1R. Below these maxima, the results could be fitted to simple laws of the formC=A_ln |T−TN−| +B_, but above the maxima a logarithmic form could only be fitted by choosingTN+more than 5×10−3°K higher thanTN−. Even then the agreement was not very good, and an apparently better fit could be obtained using the law of the formC=A′(T−TN)−&agr;+B′ withTN+=TN−and &agr;=0.31±0.02. This value of &agr; would be considerably larger than predicted by any of the present theories. These experiments suggest that small broadening effects present even in apparently homogeneous samples can lead to considerable uncertainties in the analysis of critical‐point behavior.

ISSN:0021-8979    

DOI:10.1063/1.1709704

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A neutron‐diffraction study of antiferromagnetism and the wave‐vector‐dependent susceptibility of Tb near its Ne´el temperature shows a susceptibility which can be represented over a range of temperatures by &khgr;q−1=&khgr;Q−1+ (qz−Q)2/(eq0)2+ (qx2+qy2)q02withe=2.6 andq0=0.017 (4&pgr;b3) and with a &khgr;Qwhich roughly follows the expression [(T−TN)/T1]−4/3, withT1=6°K. There is a more rapid decrease in &khgr;qthan predicted by the Lorentzian form for large values of (qz−Q) and a less rapid decrease than predicted for large values ofqx. The wave vector for which &khgr;qis a maximum shifts further away from ferromagnetism the higher the temperature.

ISSN:0021-8979    

DOI:10.1063/1.1709705

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The temperature dependence of the57Fe hfs in FeF2has been obtained with the Mo¨ssbauer effect in the region below the Ne´el temperature. In the low‐temperature region (0°−50°K) the present measurements are in good agreement with the more precise determination of the temperature dependence of the19F NMR by Jaccarino. In the region just below the Ne´el temperature the data were analyzed in terms of the equationH(T)=H(0)D(1−T/TN)&bgr;,yielding &bgr;=0.325±0.005 andD=1.36±0.03. This value of &bgr; differs slightly but not significantly from that found in MnF2by Heller.Dis larger than those previously reported in other materials because of the larger magnetic anisotropy in FeF2.

ISSN:0021-8979    

DOI:10.1063/1.1709706

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A nickel single‐crystal sphere is placed in a solenoid with a 100 axis along the solenoid axis. The solenoid is placed in an electromagnet with the field at right angles to the solenoid axis and along another 100 axis of the nickel crystal. A detector coil is placed with its axis parallel to the solenoid in order to measure the change in the component of the magnetization of the sphere along the solenoid when the 20‐G field in the solenoid is reversed. This component is measured as a function of the transverse field of the electromagnet to obtain the effective susceptibility in applied fields from 20 to 5000 G. A sufficient time is allowed between each change of the electromagnet field for the large magnetocaloric effects to decay and the sample to return to the furnace temperature. The temperature dependence of the spontaneous magnetization in zero internal field and of the initial rate of increase with field of magnetization above the spontaneous magnetization below the Curie temperature, the temperature dependence of the initial susceptibility above the Curie temperature, and the field dependence of the magnetization at the Curie temperature and at a number of nearby temperatures are obtained. The single‐crystal results show sharper indications of the spontaneous magnetization than do identical measurements on polycrystalline Ni. The difference is attributed to Dzialoshinski interactions at grain boundaries. Analytic representations of some of the data are given.

ISSN:0021-8979    

DOI:10.1063/1.1709707

出版商:AIP    

年代:1967

数据来源: AIP